1. Field of the Invention
The present invention relates to a photoconductive polyimide, an organic film comprising the polyimide, and a photoconductive device comprising the organic film, which are useful for a photoelectro-transducing device such as a solar cell or a photosensor.
2. Related Background Art
Photoelectro-transducing devices for converting light energy directly to electric energy are used for solar cells and photosensors, so that the development of a photoelectro-transducing material is significant for utilization of solar energy and development of light receiving devices in optoelectronics.
Solid state photoelectro-transducing devices which have been practicalized hitherto employ an inorganic semiconductor such as a single crystal or amorphous silicon. In contrast thereto, photoelectro-transducing devices employing an organic material, which are still in a research stage, are attractive because such devices have the general characteristics of organic materials such as light weight, ease of production, mass-productivity, and inexpensiveness, and are desired for development of organic solar cells and photosensors.
Known organic photoelectro-transducing materials include dyes such as chlorophyll, squarylium, phthalocyanine, merocyanine, etc.; and semiconductive polymers such as poly(N-vinylcarbazole), poly[4-(N,N-diphenylamino)-phenylmethyl methacrylate], etc. which are electrochemically doped, and the like. Organic thin films of these materials are being employed for development of novel photoelectro-transducing devices.
In making a practical device using an organic thin film, a high photoelectro-transducing efficiency is naturally a prerequisite, and additionally, heat stability of the device and workability of the material into a desired shape of the device are extremely important.
For the heat stability, the heat-resistance temperature should be at least 200.degree. C., preferably 300.degree. C. or higher, and a still higher heat-resistance temperature is even more preferable.
The heat resistance relates closely to the melting point, the decomposition temperature, or the glass transition temperature (Tg) of the material. These temperatures will determine the heat-resistance temperature. For example, merocyanine pigments, which are low molecular photoconductive organic materials, have a heat-resistance temperature of approximately 160.degree. C. at the highest, and poly(N-vinylcarbazole) compounds, which are semiconductive polymers, have heat resistance temperatures of around 150.degree. C., depending on the molecular weight. Polymethyl methacrylate, a general-purpose plastic material, has a glass transition temperature of approximately 100.degree. C.; polyvinyl chloride, approximately 70.degree. C.; and polystyrene, approximately 100.degree. C.
On the other hand, phthalocyanines have superior heat stability, and some of them have a decomposition temperature of 200.degree. C. or higher. However, the phthalocyanines are significantly inferior in workability, and will not readily form flat, uniform thin films, which strictly limits application of the phthalocyanines.
Accordingly, by use of an ordinary organic low molecular compound or an organic polymeric compound, it is difficult to attain a melting point, decomposition temperature, or Tg of 300.degree. C. or higher of the material for raising the heat resistance temperature without impairing the workability of the thin film.
Therefore, in developing a photoelectro-transducing device employing an organic thin film, improvement of environmental stability such as heat resistance, and thin-film workability are important in addition to the photoelectro-transducing efficiency. Nevertheless, an organic photoelectro-conductive device which has excellent electroconductive characteristics previously could not be obtained because of the above-mentioned technical difficulties.
In technical fields other than photoelectro-transducing, various heat-resistant resins are known, among which polyimide resins have excellent heat resistance. The polyimide resins are commercialized only for uses requiring heat resistance, wear resistance, chemical resistance, etc. because of the properties thereof, and the use of a polyimide for photoconductive material is still in a research stage. Polyimides having a porphyrin skeleton are also being investigated, which have heat resistance temperature of approximately 200.degree. C., which is still not sufficient heat resistance.